1. Field of the Invention
The present invention relates to a process for the production of synthesis gas.
More specifically, the present invention relates to a process for the production of synthesis gas by means of the autothermal reforming of light hydrocarbons.
Even more specifically, the present invention relates to a process for the production of synthesis gas by means of the autothermal reforming of natural gas and/or methane.
2. Description of the Background
Processes for the production of synthesis gas from light hydrocarbons have been known for a long time. According to the steam reforming technique, methane, or another light hydrocarbon, is reacted with water vapor at a high temperature according to the reaction scheme:
CH4+H2O=CO+3H2xe2x80x83xe2x80x83(I)
The reaction is considerably endothermic, requires in fact about 50 Kcal per mole of converted methane and is therefore not very convenient from an industrial point of view due to the high operating costs associated with the energy consumption.
In order to overcome these limits, an alternative technology has been proposed, known as xe2x80x9cpartial oxidationxe2x80x9d as the heat necessary for the production of synthesis gas derives from the partial oxidation of methane, according to the reaction scheme:
CH4+1/2O2=CO+H2xe2x80x83xe2x80x83(II)
With partial oxidation, moreover, significant quantities of carbon dioxide and water vapor are always formed as the methane is always partially transformed also into these products (total oxidation). The equilibrium of the xe2x80x9cshiftxe2x80x9d reaction therefore tends to be established:
CO+H2O=CO2+H2xe2x80x83xe2x80x83(III)
The high exothermicity of the partial oxidation reaction hinders industrial application as temperatures much higher than 1000xc2x0 C. are reached under adiabatic conditions, considerably increasing the investment costs relating to the materials and construction technologies.
The application of the autothermal reforming reaction has become widely used in the last few years, and is carried out by contemporaneously feeding pure oxygen, as primary oxygen source, and methane, or another light hydrocarbon, with water vapor to the synthesis reactor so that reactions (I) and (II) take place contemporaneously and the endothermicity of the one compensates the exothermicity of the other, ensuring that there is no distinct production or consumption of heat. Also in this case, however, the process is not very convenient as the use of pure oxygen as primary oxygen source requires the running of a cryogenic unit for the separation of air, whose investment and operating cost greatly jeopardizes the oxidative reforming process. In fact, in a process for the production of synthesis gas by means of autothermal reforming, more than 50% of the production cost is linked to the production of oxygen. On the other hand, it is also not very convenient to use air as such or enriched, as the nitrogen present in the air itself would dilute the synthesis gas to a degree which is not acceptable for most applications.
To overcome this latter problem, the use of metal oxides capable of being reduced by methane or another hydrocarbon, has been proposed, as primary oxygen source. In xe2x80x9cIndustrial and Engineering Chemistry,xe2x80x9d Vol. 41, Nr. 6, 1227-1237 (1949) a catalytic partial oxidation process of methane is described wherein the oxygen source consists of copper oxide which proves to be a very active oxidizing agent and which can be easily re-oxidized by means of air. The patent U.S. Pat. No. 5,799,482, in particular, describes a process for the production of synthesis gas in which the partial oxidation of a light hydrocarbon is effected continuously, to produce synthesis gas, using as primary oxygen source, a metal oxide capable of undergoing continuous reduction/oxidation (redox) cycles. Oxides cited as being particularly suitable for undergoing redox reaction cycles are oxides of copper, chromium, cobalt, iron, manganese, their mixtures or, alternatively, binary or ternary metal oxides.
The embodiment of the continuous process described in the U.S. Pat. No. 5,799,482 comprises the use of two fluid bed reactors. The first reactor (autothermal reactor), operating at a preferred temperature of 1600-1850xc2x0 F. and at a pressure of 150-450 psig, contains the metal oxide, and an optional catalyst which activates the partial oxidation reaction according to schemes (II) and (III), and is fed continuously with the light hydrocarbon to be oxidized. The second reactor (combustor/regenerator), operating at a temperature higher than that of the first, contains the reduced metal oxide and is fed continuously with a fuel mixture (air/methane) to burn the carbonaceous residues present on the solid and re-oxidize the metal.
The two reactors are connected to each other and continuously exchange the exhausted oxide and regenerated oxide. According to this process, moreover, the exhausted gases leaving the regenerator are mixed with fresh air at a high pressure and used in a gas turbine to produce energy.
The process of patent U.S. Pat. No. 5,799,482 also has its drawbacks. In fact, the reduction of the metal oxide is endothermic and, even in the case of pure catalytic partial oxidation, in order to keep the reaction temperature constant in the first reactor, heat is supplied by recycling the regenerated oxide at a temperature higher than the oxidation temperature, thus increasing the investment and operating costs. In addition, the oxidative action of the oxide must be integrated with supplementary oxidizing gas, for example pure oxygen, air as such or enriched air, thus conserving, although in reduced form, the disadvantages of the previous technologies.
An objective of the present invention is to provide a process for the production of synthesis gas by means of catalytic partial oxidation or autothermal reforming in which the primary oxygen source is represented by a metal oxide capable of undergoing redox cycles which does not have the drawbacks described above.
More specifically, the objective of the present invention is to provide a process for the production of synthesis gas which is actually autothermal and which consequently does not require any heat supplement, supplied externally for example, by means of the regenerated and recycled metal oxide, and in which the primary oxygen source is exclusively represented by a metal oxide capable of undergoing redox cycles, without any additional oxidative sources.